Classification of the Epilepsies
Tobias Loddenkemper
Hans O. Lüders
Imad M. Najm
Elaine Wyllie
The most widely used system for classification of epilepsies was proposed in 1989 by the Commission on Classification and Terminology of the International League Against Epilepsy (ILAE) (1). ILAE’s 1981 seizure classification (2) and its 1989 classification of the epilepsies (1) have given physicians around the world a common language. The ILAE classifications were based mainly on two features: (a) distinction between generalized and focal seizure types and (b) etiologic considerations. This classification, like all other classification systems, is not without its shortcomings, and new approaches have been proposed.
THE 1989 ILAE CLASSIFICATION
The 1989 ILAE epilepsy classification is used world-wide and is reproduced in the appendix to this chapter. It was revised from proposals made in 1970 (3) and 1985 (4), and, like the 1981 ILAE seizure classification (2), is based primarily on the definition of electroclinical syndromes. In 1969, Henri Gastaut proposed the first classification of epilepsies (5), which was used as the basis of the first ILAE epilepsy classification system that was proposed 1 year later (3). This classification provided the major division between “partial” (focal) and generalized epilepsies. Each seizure type was grouped according to this dichotomy and associated with interictal and ictal electroencephalographic (EEG) findings, etiology and pathologic findings, and age of manifestation.
About 15 years later, a revision introduced the concept of epilepsy syndromes “defined as an epileptic disorder characterized by a cluster of signs and symptoms customarily occurring together. The signs and symptoms may be clinical (e.g., case history, seizure type, modes of seizure recurrence, and neurological and psychological findings) or as a result of findings detected by ancillary studies (e.g., EEG, x-ray, CT [computed tomography], and NMR [nuclear magnetic resonance])” (4). This revision divided many specific epilepsy syndromes under the major dichotomy of generalized and “localization-related” (focal) epilepsies and associated them with clinical and EEG findings, etiologies, and disease severity.
The primary dichotomy of these classification systems was set between localization-related (focal) epilepsies, “in which seizure semiology or findings at investigation disclose a localized origin of the seizures” (1), and generalized epilepsies, characterized by “seizures in which the first clinical changes indicate initial involvement of both hemispheres…[and] the ictal encephalographic patterns initially are bilateral” (1). EEG findings are the laboratory results that carry the most weight for defining a focal epilepsy syndrome.
In addition to localizing information, previous epilepsy classifications also contained etiologic information. The 1970 epilepsy classification (3) further divided the generalized epilepsies into primary—those occurring in the setting of normal neurologic status, with seizures that begin in childhood or adolescence and lack any clear cause—and secondary—those involving abnormal neurologic or psychological findings and diffuse or multifocal brain lesions. Because the term secondary generalized epilepsy was sometimes confused with the different concept of “secondary” or “secondarily” generalized tonic-clonic seizures, it was abandoned in the 1985 (4) and 1989 (1) revisions. Primary and secondary were replaced with idiopathic and symptomatic. The 1970 classification (3) applied the etiologic dichotomy only to generalized epilepsies because all focal epilepsies were assumed to be associated with some type of brain lesion. This neglected the idiopathic syndrome of benign epilepsy of childhood with centrotemporal spikes, and therefore the 1985 (4) and 1989 (1) revisions applied idiopathic and symptomatic to the focal epilepsies as well. The term cryptogenic was added in the 1989 (1) classification to describe epilepsy syndromes that are presumed to
be symptomatic but are of unknown cause in specific patients.
be symptomatic but are of unknown cause in specific patients.
2001 ILAE PROPOSAL: A SYNDROME-ORIENTED CLASSIFICATION
To resolve existing controversies, the ILAE’s Commission on Classification and Terminology published a common terminology for ictal semiology (6) and a revised five-axis classification scheme of epilepsies (7). This proposal (7) is again based on epilepsy syndromes that appeared in previous classifications. The author defines an epileptic syndrome as “[a] complex of signs and symptoms that define a unique epilepsy condition” (7). In this classification, the epilepsy syndrome represents the third of five axes of the ILAE proposal and is separated from the epileptic disease, which consists of “a pathologic condition with a single specific, well-defined etiology. Thus progressive myoclonic epilepsy is a syndrome, but Unverricht-Lundborg is a disease” (7).
Axes of the 2001 ILAE Proposal
The different axes in the 2001 ILAE proposal include seizure description (axis 1), seizure type (axis 2), epilepsy syndrome (axis 3), etiology (axis 4), and impairment (axis 5) (Table 22.1). Details are available online at http://www. epilepsy.org/ctf.
Axis 1 describes ictal seizure semiology through a standardized glossary of descriptive ictal terminology (6). This terminology is independent of pathophysiologic mechanisms, epilepsy focus, or seizure etiology.
Axis 2 is based on a list of accepted epileptic seizure types constructed by the task force. These seizure types are closely related to diagnostic epilepsy entities or indicate underlying mechanisms, pathophysiology, or etiology, or implicate related prognosis and therapy. The list is divided into continuous and self-limited seizure types and includes precipitating stimuli for reflex seizures. Self-limited and continuous seizure types are further divided into seizures of generalized and focal origin.
TABLE 22.1 KEY FEATURES OF THE 2001 ILAE CLASSIFICATION AND THE PATIENT-ORIENTED EPILEPSY CLASSIFICATION | ||||||||||||
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Axis 3 identifies the epilepsy syndrome diagnosis and separates epilepsy syndromes from entities with epileptic seizures. Epilepsy syndromes are divided into “syndromes in development” and fully characterized syndromes (7).
Axis 4 delineates the etiology of epilepsies, which includes pathologic and genetic causes as well as diseases frequently associated with epilepsy. The ILAE Task Force (7) is revising and updating this list of diseases.
Axis 5 will include an optional classification of the degree of disability and impairment caused by the epilepsy and will be based on the revised International Classification of Functioning, Disability and Health (ICIDH-2) (http://www.who.int/icidh).
Advantages of the 2001 ILAE Proposal
Diagnostic Scheme
Compared with the 1989 version of epilepsy classification, the diagnostic scheme of the 2001 proposal is an attempt to overcome shortcomings and confusion among EEG features, clinical seizure semiology, and syndromatic classification efforts. By dividing the seizure classification into several axes, the ILAE responds to the criticism that a strict one-to-one relationship is lacking between epilepsy syndromes and seizure types. The introduction of a multiaxial diagnostic scheme reflects the recognition of epilepsy as a clinical symptom that can manifest with different semiologic seizure types and be intertwined with different etiologies. Further disentanglement of clinical presentation, etiology, and disease severity is a long-anticipated milestone that will facilitate additional clinical and research applications of the proposed diagnostic scheme.
Emphasis on Seizure Semiology
By including an axis that is based solely on seizure semiology, the ILAE moves away from the electroclinical approach to epilepsy syndromes in the 1989 proposal. The diagnostic multiaxial scheme shifts focus from alteration of consciousness to other semiologic features, thus placing greater emphasis on the symptomatology of seizures that may carry localizing and lateralizing information.
Breaking Up the Dichotomy Between Generalized and Localization-Related Epilepsy
The diagnostic multiaxial scheme pays more attention to the spectrum between previously described “localization-related” epilepsies and generalized epilepsies. This is
reflected in the replacement of the expression “partial,” which has been used for epilepsies as well as seizures and the never-really-established term localization related, by “focal.” The regression to the older but more established terminology facilitates communication. The broadened meaning of focal will now involve more widespread areas of cortical dysfunction.
reflected in the replacement of the expression “partial,” which has been used for epilepsies as well as seizures and the never-really-established term localization related, by “focal.” The regression to the older but more established terminology facilitates communication. The broadened meaning of focal will now involve more widespread areas of cortical dysfunction.
Syndromes Are Not Fixed Entities
Compared with the 1989 ILAE classification, the 2001 proposal’s diagnostic scheme addresses the criticism that syndromes are not fixed entities and introduces the concept of accepted syndromes versus syndromes in development. This separation allows for the incorporation of new scientific findings and entities into the classification system.
Advances and Clarifications in Terminology
The expression cryptogenic is replaced by the more self-explanatory term probably symptomatic. Additionally, the avoidance of the word convulsions will decrease miscommunications and facilitate more exact seizure description.
Limitations of the 2001 ILAE Proposal
Epilepsy Syndrome Concept
The 2001 ILAE proposal (7) defines an epilepsy syndrome as “[a] complex of signs and symptoms that define a unique epilepsy condition. This must involve more than just the seizure type: thus frontal lobe seizures per se, for instance, would not constitute a syndrome.” Syndromes variously combine a constellation of findings including seizure semiology, electrophysiologic ictal or interictal findings, pathologic-anatomic substrate, etiology, imaging findings, age of onset, related medical conditions, and medical history. Seizure types are not unique to etiologies and may be associated with many different clinical, radiologic, and electrophysiologic features leading to a wide variety of possible syndromic combinations. The 2001 proposal lists more than 50 epilepsy syndromes. The currently recognized syndromes provide the clinician with limited reliable information on prognosis, outcome, and etiology owing to heterogeneity within each syndrome. This can be attributed to the fact that syndromes are classified according to varying inclusion criteria.
Epilepsy Syndromes as a Function of Age
Gibbs, Gibbs, and Lennox (8) note in their historic article, “Epilepsy: A Paroxysmal Cerebral Dysrhythmia,” that “[d]iseases change their names with increase, not of age (like Chinese children), but with the increase of medical knowledge. Most disorders begin life bearing the name of the man who first recognized them or else they are temporarily tagged with some purely descriptive term. When the etiology or pathology is discovered, the nomenclature is changed.” According to the ILAE’s syndrome-oriented classification, the same patient can bear several epilepsy syndrome diagnoses at different ages. Infants initially diagnosed with Ohtahara syndrome can later be described as having West syndrome, Lennox-Gastaut syndrome, and then undefined generalized epilepsy. If epilepsy syndromes reflect biologic entities, the diagnosis should be affected only by increase in medical knowledge and not by the patient’s age.
Application of the ILAE Epilepsy Syndrome Classification
Studies indicate that in a general family practice, only 5% of all patients can be classified according to the ILAE syndromes (9); in a general neurology practice, 11% of all epilepsy patients can be classified according to the ILAE syndromes (10); and epileptologists can sort only 25% of all patients into ILAE epilepsy syndrome categories (11). These studies indicate that more patients can be classified as further information becomes available in each case and the more skilled the classifying physician is. Nevertheless, 75% of all patients are not classifiable according to the ILAE syndrome-oriented classification even by fully trained epileptologists, indicating that the majority of epilepsy patients do not fit any syndromic category. The 2001 ILAE proposal recognizes this flaw (7).
ILAE Classification Axes Overlap
Terminology in the first three axes of the ILAE proposal mingles seizure semiology and epilepsy syndromes, as demonstrated by the following example: A 3-year-old patient with daily myoclonic seizures (20 per day) followed by astatic seizures frequently triggered by light and photic stimulation would be classified according to the ILAE system as:
Axis 1 (ictal semiology)—myoclonic-astatic seizure
Axis 2 (pathophysiologic seizure types)—myoclonicastatic seizure
Axis 3 (epilepsy syndrome)—epilepsy with myoclonicastatic seizures
Axis 4 (etiology)
Axis 5 (impairment)
FIVE-DIMENSIONAL PATIENT-ORIENTED EPILEPSY CLASSIFICATION PROPOSAL
An alternative patient-oriented epilepsy classification (12) was developed in an effort to address some of the difficulties with the 2001 ILAE proposal (7). The five dimensions can be used to capture the critical features in patients with epilepsy. However, at the first encounter, it is sometimes difficult to determine whether a patient is suffering from epilepsy or is having nonepileptic seizures. Therefore, early in the investigation, the term paroxysmal event may be used to indicate the uncertainty of an epilepsy diagnosis. As more information becomes available (e.g., observations by
witnesses, routine EEG recordings, magnetic resonance imaging [MRI] of the brain, and video-electroencephalography), the classification can become more specific. If these results indicate that the patient has nonepileptic seizures, other classification systems can further characterize the event (13).
witnesses, routine EEG recordings, magnetic resonance imaging [MRI] of the brain, and video-electroencephalography), the classification can become more specific. If these results indicate that the patient has nonepileptic seizures, other classification systems can further characterize the event (13).
Dimensions of the Patient-oriented Classification
Dimension 1: The Epileptogenic Zone
The first dimension characterizes the localization of the epileptogenic zone, as determined by all available clinical information (e.g., history, examination, electroencephalography, MRI). Classification is recognized as an ongoing interactive process with an increasing degree of precision as additional clinical data become available. If it is uncertain whether the patient has epilepsy or nonepileptic seizures, the term paroxysmal event is used. As further information becomes available (e.g., an electroencephalogram demonstrating left mesial temporal sharp waves, left temporal electroencephalographic seizures, and an MRI showing left hippocampal atrophy), the classification becomes more precise (left mesial temporal lobe epilepsy). If the patient has epilepsy but the epileptogenic zone cannot be determined further, the expression unclassified epileptogenic zone is used. If additional localizing evidence is available, a subcategory such as focal, multifocal, multilobar, generalized, or other is used. The categories focal, multilobar, and multifocal allow for further specification (Table 22.2) (14). Multifocal indicates more than one epileptogenic zone in different lobes. The term generalized is used if the cortex is diffusely epileptogenic without a localizable epileptogenic zone. Further characterization of the localization of the epileptogenic zone is possible by the addition of “left” or “right.”
The traditional ILAE epilepsy syndrome (if applicable) can be added in parentheses after the epileptogenic zone (e.g., Rasmussen encephalitis) to provide clinicians with traditionally used key words. However, the five dimensions of this classification contain all the information necessary to identify ILAE syndromes.
Single seizures and situation-related seizures (e.g., febrile seizures and seizures induced by electrolyte or metabolic disturbances) can also be classified within certain limits by this system, but classification of these disorders is not the primary intention of this epilepsy classification.
Dimension 2: Seizure Classification
The clinical signs and symptoms are the most important pieces of information for localizing a lesion in the central nervous system. Seizures and seizure semiology are the clinical manifestation of epilepsy. A seizure classification based solely on this clinical presentation of the epilepsy has been used successfully at several centers (15, 16, 17, 18, 19).
TABLE 22.2 CLASSIFICATION OF THE EPILEPTOGENIC ZONE (FIVE-DIMENSIONAL EPILEPSY CLASSIFICATION) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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This seizure classification uses only the clinical semiology and does not require any additional diagnostic techniques other than analysis of an observed or videotaped seizure. Information from MRI, EEG, or positron emission tomography (PET) is unnecessary. The semiologic seizure classification distinguishes among auras, autonomic seizures, dialeptic seizures (characterized primarily by loss of awareness), motor seizures, and special seizures such as atonic, hypomotor, and negative myoclonic seizures (17, 18, 19).
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